TrxG

TrxG, or Trithorax Group, refers to a conserved set of chromatin-modifying proteins and complexes that maintain transcriptional activation of specific genes, especially developmental regulators such as homeotic genes. In cellular memory, TrxG acts in opposition to Polycomb Group (PcG) repressors to preserve active chromatin states across cell divisions.

Biochemically, TrxG promotes gene activation through multiple activities, including histone modifications associated with open chromatin. Key among these is methylation of histone H3 on lysine 4 (H3K4) by SET1/MLL-family methyltransferases, along with histone acetylation and chromatin remodeling. These modifications help keep target loci accessible and transcription-ready.

TrxG activity is organized into several multi-protein complexes with distinct enzymatic functions. Among them are H3K4 methyltransferase complexes and various coactivator assemblies. While exact composition varies across species, the TrxG network is broadly conserved from invertebrates to vertebrates and operates to sustain active transcriptional states.

Biological significance of TrxG lies in proper development and cell fate determination. Loss or disruption of TrxG components can lead to misexpression of developmental genes and homeotic transformations, reflecting its role in maintaining stable gene expression programs. Aberrant TrxG function has been linked to diseases, including cancer, where epigenetic regulation is perturbed.

Historically, TrxG and PcG were identified in Drosophila as opposing regulators of HOX gene expression, establishing a paradigm of epigenetic memory that helps explain how gene expression patterns are inherited through cell generations.